The invention relates to a wiper device for a vehicle windshield. Prior art (U.S. Pat. No. 2,505,078) discloses a device with reciprocating wipers in which a median plane that passes through a wiper blade extends perpendicular to the windshield when the wiper blade placed on the windshield is not loaded. Such a blade is able to optimally clean a planar windshield. The relationship between the wiping blade and the windshield does not change over the windshield area covered during reciprocal movement of the blade, and no deviation of the wiping blade from the normal line occurs. When the windshield has a curved surface, the angle of inclination of this median plane, to the outer surface of the windshield constantly changes over the area covered by the reciprocating blade. This change is also noticable in the longitudinal direction of the wiping blade which, upon its reciprocal movement describes a ring segmental area. The position of the plane relative to the windshield area traversed thereby, at the outer sides of the arc of the described area, changes as at the inner arc so that at the middle arc described by the link between the wiping blade and a wiper arm. The optical orientation of the plane relative to the windshield achieved when the windshield has a planar surface, can only be achieved pointwise when the windshield has a curved surface, and it constantly changes in the longitudinal direction of the wiping blade. The actual deviations from an ideal orientation will be greater in more curved areas of the windshield than in the base - middle areas thereof. Flexibility of a contact element of the wiping blade can compensate these deviations only to a certain extent.
Therefore, in known wiper devices the oscillation axis is so oriented relative to the windshield contour that, in the middle position of the wiping blade, the oscillation axis would extend ideally to the windshield area passing therethrough in the next following position of the median plane. The more curved is the windshield in the covered area, the more the position of the median plane deviates from the ideal position. This deviation error is called normal deviation and is expressed as an angle. As discussed above, the normal deviation is always large at inner, middle and outer circles. From the above, it follows that the angle defining the normal deviation is formed by a line lying in the described plane and a line which is perpendicular to a windshield surface which passes through the line lying in the described area. Another drawback consists in that the foregoing deviation from an ideal condition changes with movement of the wiping blade in opposite directions. It is especially difficult to control the conditions which occur in so-called one-arm windshield-wiper systems where the wiping blade has to cover the both strongly curved areas of the windshield.
DE-OS 3,638,159 discloses a device in which there is provided means for transforming an oscillating movement of a wiper arm into pivotal movement of the wiping blade. However, this transforming means is rather complicated and should be made with a very high precision.